Typically, in the production of emulsion polymers by free-radical polymerization, it has been a common practice to use nonionic or anionic materials to stabilize the emulsions. Alternatively, others have used protective colloids like poly(vinyl alcohol) [PVOH], hydroxyethyl cellulose, or derivatives thereof, either singly or in combination, to stabilize aqueous emulsion polymers. These stabilizers produce emulsion particles that are electrically neutral or negatively charged in an aqueous environment.
Commercially produced emulsions that contain positively charged particles (that is, cationic emulsions) are known, but are far fewer in number. For example, U.S. Pat. No. 5,521,266 to Lau discloses the synthesis of cationic emulsions, but requires complexation of water-insoluble monomers with macromolecular carbohydrates (for example, cyclodextrin, cyclodextrin derivative, cycloinulohexose, cycloinuloheptose, cycloinuloctose, calyxarene and cavitand) having a hydrophobic cavity.
U.S. Pat. No. 4,308,189 to Moritani et al. reviews many conventional techniques for producing cationic emulsions using low-molecular-weight cationic emulsifiers such as laurylamine salt, octadecylamine salt, laurylpyridinium chloride, and others that are toxic and subject to stringent regulations. The '189 patent also describes the use of cationic initiators and monomers to create cationic emulsions.
U.S. Pat. No. 5,254,631 to Yamamoto et al. discloses cationically electrodepositable, finely divided gelled polymers (that is, internally cross-linked polymer particles that do not coalesce into a film upon drying) having a core-sheath structure obtained by emulsion polymerization.
So, there are few processes that yield stable emulsions and none that offer satisfactory performance properties in applications that require film formation on and adhesion to plastic film or adhesion to inks after prolonged exposure to water or solvents like isopropyl alcohol (IPA).
For example, U.S. Pat. No. 4,214,039 to Steiner et al. discloses a cationic polymer as a primer for vinylidene chloride polymers used as coatings for oriented polypropylene packaging films. The primer comprises an epoxy resin composition comprising a) a liquid epoxy resin, e.g., one based on Bisphenol A, preferably emulsified or dissolved in water, and b) a water-soluble, amine-modified acrylic resin. This system, also employed at higher coating weights in U.S. Pat. No. 6,025,059 to McGee et al., lacks shelf stability. Once the ingredients are mixed, the ingredients start to react. The useful pot life of the mixture of the '059 patent is no more than about 3 days. After this, the mixture gels or agglomerates, with precipitation of components. Moreover, undesired blocking can occur at coating weights below 0.25 grams/1000 in2 (g/msi). In addition, ink adhesion problems can occur during printing with black UV-curable screen ink. Finally, the formulation may contain amounts of up to 10-20 wt. % propylene glycol monomethyl ether, which may require certain precautions in handling and use on a commercial scale.
Typically, films prepared for use as label facestock are coated on the printing side with a coating, which enhances ink adhesion. For instance, U.S. Pat. No. 5,380,587 to Musclow et al. discloses a multilayer packaging or label stock film having excellent printability and non-blocking characteristics. The film is first primed and then coated with copolyester coating.
Another ink adhesion enhancing coating is described in U.S. Pat. No. 5,382,473 to Musclow et al. which discloses a multilayer film structure with a prime coating which is the reaction product of acidified aminoethylated vinyl polymer and epoxy resin, top coated with polyester ink base to eliminate blocking.
U.S. Pat. No. 5,662,985 to Jensen et al. discloses a two-side coated label which comprises a polymeric film substrate having on a first surface thereof (A) an adhesive anchor layer and on a second surface thereof (B) an ink base layer, the (A) and (B) layers being selected from the group consisting of: (i) a prime coating having on an external surface a functional coating of an interpolymer of (a) an alpha, beta-monoethylenically unsaturated carboxylic acid; and (b) a neutral monomer ester comprising an alkyl acrylate ester and an alkyl methacrylate ester; and (ii) an iminated polymer; or the (A) adhesive anchor layer being selected from the group consisting of: (iii) a mixture of the functional coating of (i) and the iminated polymer of (ii); (iv) a linear water dissipatable polyester condensation product; and (v) a polyester; or the (B) ink base layer being selected from the group consisting of: (vi) a prime coating having on an external surface a functional coating of an acrylic copolymer; and (vii) a prime coating having on an external surface a functional coating of a styrene copolymer, provided that each of the (A) adhesive anchor layer and the (B) ink base layer is different. This invention offers excellent adhesion to most inks, but lacks resistance to IPA and blushes when the coated film is exposed to hot water.
One-package aqueous “latices” containing alkaline-curable self-cross-linking polymers are disclosed in U.S. Pat. No. 4,546,140 to Shih. These cationic polymer emulsions were stabilized by cationic monomers and nonionic surfactants. Shih's invention requires the presence of a salt of an organic carboxylic acid to cure polymerized acrylic esters containing a halohydrin and/or a quaternary ammonium salt. Shih's emulsions do not contain epoxy-functional monomers.
U.S. Pat. No. 5,296,530 to Bors et al. discloses polymers containing pendant acetoacetoxy groups maintained at relatively high pH values of >9 prior to application on a substrate.
U.S. Pat. No. 5,525,662 to Lavoie et al. discloses nonionic and anionic stabilizers for polymers containing acetoacetoxy-functional polymers. This reference teaches that unless the acetoacetoxy moiety is converted into an enamine in the wet state (at pH values >9), the acetoxy group will hydrolyze at any pH, especially when heated.
U.S. Pat. No. 5,811,121 to Wu et al. relates to pH-sensitive coatings comprising cellulose acetoacetate esters used in controlled release of active agents, e.g., in pharmaceuticals. Extrapolation of data by Wu et al. (U.S. Pat. No. 5,811,121) shows that at pH 7, acetoaceoxy-functional cellulose will have the acetoxyacetoxy group completely hydrolyzed within a week at pH 7. In many applications, however, it would be useful to provide an acetoacetoxy-functional coating that is stable at room temperature for at least several weeks at pH values between 4 and 7.
U.S. Pat. No. 5,498,659 to Esser discloses water-based polymeric formulations in which the polymer contains acetoacetoxy functionality and requires the use of a non-polymeric amine and a polymeric component that has both acid-functional and acetoacetoxy-type functional moieties.
U.S. Pat. No. 6,297,328 to Collins et al. discloses a surfactant-containing, enamine-functional polymer comprising the reaction product of a surfactant-containing acetoacetoxy-functional (AcAc) polymer and ammonia, a primary amine, a secondary amine or a poly(alkylenimine), for example PEI, that is, poly(ethylene imine), or other amines that are water-soluble and stable at pH values >8. Collins et al. disclose a surfactant-containing acetoacetoxy-functional polymer which is a reaction product of a non-acid vinyl monomer having an acetoacetoxy-type functionality, for example acetoacetoxy ethylmethacrylate (AAEM), and at least one non-self polymerizing, ethylenically-unsaturated surfactant monomer. The disclosure further recommends preparing AcAc polymer emulsion in the presence of nonionic and anionic emulsifiers.
All of the foregoing U.S. patents are incorporated herein by reference.
The development of commercially acceptable coated plastic films for printing applications, e.g., printable labels, is often a compromise between a variety of desired properties. Labels used for beverage containers, or health and beauty containers, should be capable of exposure to severe conditions encountered during manufacturing, transport and storage. Thus printable coatings for plastic films should exhibit hot-water resistance, organic-solvent resistance, e.g., IPA resistance, abrasion resistance, and haze resistance on exposure to hot or cold water. At the same time, the coating should be receptive to ink so as to provide good adhesion of the ink to the coated film immediately after printing. The ink should stay adhered to the coated film after a label is made and applied to a beverage container that is exposed to hot or cold water and subsequent abrasion encountered in mechanized handling.
Non-cross-linked polymer constituents of coatings tend to increase in haze upon exposure to boiling water and may be completely soluble and removed upon exposure to IPA. In addition, after ink is applied and the label exposed to water and abrasion simulating a bottling line, the ink will abrade off the label. Coatings can be made resistant to hot water or chemicals by cross-linking polymers in the coating. However, when cross-linked, coatings are generally less receptive to inks, losing the ability to have good ink adhesion immediately after printing, especially at high printing speeds and low temperature. Even where such coatings are resistant to IPA and show robust printing performance with a wide variety of inks, including UV-curable inks, field testing has revealed certain shortcomings, e.g., suitability after aging. This is particularly true where the coated film samples are heat-aged as opposed to being aged at ambient temperature. Heat aging is associated with increased cross-linking.
It would be highly desirable to provide stable cationic emulsions that allow sufficient design flexibility to create useful coatings for plastic film, e.g., a coating composition that exhibits a chemically resistant, printable surface such that the coated product can be used as a label that exhibits both acceptable solvent resistance properties and ink adhesion properties, particularly adhesion to UV-curable screen inks and electron beam-curable (EB-curable) screen inks. Moreover, it would be desirable that such a coating composition resists blocking at low coating weights, which are economically favorable. Furthermore, it would be desirable to have stable cationic emulsion polymers that could be formulated with additives to enhance adhesion to particular substrates or inks or to impart color, texture (a matte finish or paper-like appearance), anti-static properties, and/or security features. Finally, it would be useful to provide such polymer coatings that provide good ink adhesion even after heat aging.